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Problems with melting furnace


#1

Fellow orchidians,

I purchased an Itialian made melting furnace, Model FE-10, unknown
manufacturer. This is similar to an electro melt. It uses a graphite
crucible with a groove cut in it and comes with a pair of tongs that
fit into the groove and contact 90% of the groove. To pour it is
intended that the crucible be removed. The melter has a thermocouple
and a Techologic electronic temperature controller.

To get familiar with the melter I decided to pour an ingot. I heated
the sterlling to 1700 deg F, added a pinch of flux, stirred the
silver with a graphite rod, picked up the crucible with the tongs
and poured. Initially nothing happened so I tipped the crucible a
bit more. The silver poured out all at once. Only part of it made it
into the ingot mold. I had noted that when I picked up the crucible
which was a cherry red, the top part lost its color because the
tongs sucked the heat out of it. I am guessing that the initial
silver hardened and when I tipped the crucible further the melt went
over the bump.

Can anyone give me some guidance on this?


#2

I’m sorry noone has answered your inquiry yet, but I too am
interested in why this would happen. Could someone knowledgable about
electro-melt furnaces chime in?

Fellow orchidians,

I purchased an Itialian made melting furnace, Model FE-10, unknown
manufacturer. This is similar to an electro melt. It uses a graphite
crucible with a groove cut in it and comes with a pair of tongs that
fit into the groove and contact 90% of the groove. To pour it is
intended that the crucible be removed. The melter has a thermocouple
and a Techologic electronic temperature controller.

To get familiar with the melter I decided to pour an ingot. I heated
the sterlling to 1700 deg F, added a pinch of flux, stirred the silver
with a graphite rod, picked up the crucible with the tongs and poured.
Initially nothing happened so I tipped the crucible a bit more. The
silver poured out all at once. Only part of it made it into the ingot
mold. I had noted that when I picked up the crucible which was a
cherry red, the top part lost its color because the tongs sucked the
heat out of it. I am guessing that the initial silver hardened and
when I tipped the crucible further the melt went over the bump.

Can anyone give me some guidance on this?


#3

Your metal was too cold, it froze on the way out of the crucible. To
pour molten metal you need to have what is called superheat. This is
the amount of heat above the liqudius temperature of the alloy
(1640F for sterling) necessary to sucessfully pour the metal. The
correct amount of superheat will vary depending on the particular
process and equipment being used. Try 1750F or even 1800F

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


#4

Vera,

To get familiar with the melter I decided to pour an ingot. I
heated the sterlling to 1700 deg F, added a pinch of flux, stirred
the silver with a graphite rod, picked up the crucible with the
tongs and poured. Initially nothing happened so I tipped the
crucible a bit more. The silver poured out all at once. Only part
of it made it into the ingot mold. I had noted that when I picked
up the crucible which was a cherry red, the top part lost its color
because the tongs sucked the heat out of it. I am guessing that the
initial silver hardened and when I tipped the crucible further the
melt went over the bump. 

Two misconceptions exist with these italian furnaces. one is that
the metal itself is at the temp indicated by the display. That temp
is measured by a thermocouple outside the bottom of the crucible.
Often, the crucible and metal lag behind the indicated temp. To be
accurate, you need to allow the furnace to reach the setpoint temp,
and stay there for at least several minutes, before you add the metal
to be melted. The temp shown will drop a bit, then rise back up, and
again, you should let it sit there for a couple minutes after it
appears fully melted. This effect is greater with the italian type
furnaces than with the electromelts, since with the electromelts there
is a hollow cavity milled into the bottom of the crucible into which
the thermocouple fits, so the temp measured on those more closely
reads the current crucible temp, with somewhat less lag time. Not a
bit difference, but worth noting. Personally, I prefer the italian
furnaces even so, as I feel they’re more robust, and the simpler
shape crucibles last a little longer and break less easily.

The second thought is the temp you use. 1700 is not quite hot
enough. You want a temp that’s a significant interval higher than the
actual stated melting temp of the metal. At least a hundred degrees,
and for some alloys, 200 degrees higher than the melting point is
better. Experience will be your guide here. Even if the top lip of
the crucible weren’t a bit cooler (and this will be the case with
almost any type of crucible, even torch melting in a ceramic
crucible), the metal is still cooling all through the pour and into
the mold, so you have to start with a high enough temp so the metal
stays liquid until it’s fully entered the mold. I’d try 1800. If you
get the same problem, raise the temp more. 1850 might be the best
setting. And if you’re casting thin or delicate items, you might need
even more. This type of temp specification, by the way, is properly
called a “superheat”, and is considered an essential parameter of
casting methods. You’re not going to hurt the metal this way.
Especially not an alloy like sterling, which doesn’t have volatile or
consumable deoxidizers added.

The bottom line is simply that your metal wasn’t not enough.

Also, be aware that graphite crucibles with a furnace that covers
the top of the crucible create a wonderfully reducing atmosphere for
melting. There is little to any oxidation taking place, so
accordingly, you need little if any flux. Just the tinyest bit. Any
more, and you’ll find flux inclusions on your casting or ingot
surface, since unlike a clay crucible, the flux won’t be sticking to
the crucible and staying behind. At least, not as much.

Also, remember that graphite is a wonderful conductor of heat. When
you stir the melt with a graphite rod, you chilled the heck out of
the metal. You should not need to stir it like that. If you like, you
can jiggle the crucible a bit to be sure it’s fully liquid. The
bottom of the crucible tends to be the hottest area, so if the top
appears fully melted, and the temp indicator has been at the desired
temp for a few minutes, the likelyhood that the bottom is not fully
melted is slim. But again, waiting a little after it appears to melt
helps this. Then don’t stir the melt. Ordinary convection within the
liquid metal should have nicely stirred the mix anyway for a uniform
alloy. If you DO choose to stir the melt, assume you’ve just dropped
the metal temp a hundred degrees or so in doing so. The indicated
temp won’t show this until the change has transferred to the
crucible, and then changed the temp in the heating chamber. That
time lag thing again, in reverse. So if you stir, let it sit again
for a couple minutes to be sure it’s back up to the desired temp
before pouring.

Pouring these things also takes a bit of practice. The crucibles are
"long", so it’s all too easy to pour it so that the metal gets a good
running start from the bottom of the crucible and comes out the top
rather fast, causing you to miss your aim. Larger melts are easier to
pour, since you’re not tipping the crucible quite as far to get it
started.

The other hint to offer is to note that lifespan of the crucibles is
limited. Don’t expect them to last forever. In our shop, we use these
for melting all our white gold alloys for casting, which is several
melts/pours per casting session, several times a week. Normally, a
crucible lasts us four to six weeks this way. So order a spare to have
on hand. The outsides of the crucibles especially, burn away. When it
gets too too thin, change it out. this is normal for graphite
crucibles. You can get a somewhat pricey spray paint to coat the
crucibles with, which substantially slows the oxidation of the
crucibles, extending their life. I’ve not used it, so I don’t know if
it’s worth the cost. Only the outside is coated, so the crucibles
still oxidize some from the inside anyway. (this is what protects the
metal during melting, so it’s not all a bad thing. It creates that
wonderful reducing atmosphere in the metal’s environment for a clean
melt, and why you don’t need much melting flux with these things.

HTH
Peter Rowe


#5

Hi Vera,

Thanks for putting my query back on the table. I have not had an
opportunity to do any further tests so I am still in the same
situation.

Martin Buchholtz
mbuchholtz@ieee.org
727-367-6387


#6

The upper ring of the crucible must be heated. Just because the gage
indicated melt temperature does not mean the ring is at the same
temperature. The ring can be cool enough to chill the metal as it is
poured. The tongs used to hold the crucible will also chill the ring
slightly.

Use a higher temperature to melt the metal and let the unit sit for
several minutes at the melt temperature before pouring.

Originally I was using scrap sterling from a bead punching process
in my electric furnace. No matter how I cut up and bent the scrap it
would not fill the crucible when melted. I would continually add
scrap to the melt until the crucible was full of melted metal. This
allowed thr upper ring to reach the melt temperature.

Once the metal was melted I would pour. Never had a problem with the
metal chilling. When I switched to scrap pellets I did not have to
continually add scrap to the melt to fill the crucible. I would pour
as soon as the gage indicated the melt temperature was reached. The
upper ring was not given the time to heat up sufficiently even though
I was melting at the same temperature. I was having trouble getting
sharp castings after I switched to the casting pellets. I would end
up with chilled stream of metal the hanging from the crucible.

I now allow the melt furnace to stand for several minutes after it
reached the melt temperature. If you pour more than one flask from a
filled crucible you should let the crucible heat up again before
making the second pour. If you raise the lid to look at the condition
of the melted metal let the unit sit for several minutes before
pouring. Raising the lid will allow the ring of the crucible to
chill.

You should not need flux in a graphite crucible. If you use a
graphite rod to stir the metal you should let the unit heat for
several minutes before you pour. The rod will cool the metal.

Lee Epperson


#7

Peter,

Thanks so much for your insights relative to the melting and pouring
process. I still think that I need to reduce the amount of heat
transfer from the crucible lip. I was thinking of an arrangement
whereby the lower lip of the flask (the one containing the pouring
groove) could be supported at two points some distance from the
pouring groove. The object of this would be to allow the area that
the melt passes over to retain more heat. I will try your suggestions
before I get into any of this.

Thanks again,
Martin Buchholtz